Signalling by the global regulatory molecule ppGpp in bacteria and chloroplasts of land plants

Abstract

The hyperphosphorylated guanine ribonucleotide ppGpp mediates the stringent response in bacteria. Biochemical and genetic studies of this response in Escherichia coli have shown that the biosynthesis of ppGpp is catalysed by two homologous enzymes, RelA and SpoT. RelA is activated in response to amino acid starvation, and SpoT responds to abiotic physical stress beside nutritional stress. All free-living bacteria, including Gram-positive firmicutes, contain RelA-SpoT homologues (RSH). Further, novel ppGpp biosynthetic enzymes, designated small alarmone synthetases (SASs), were recently identified in a subset of bacteria, including the Gram-positive organism Bacillus subtilis, and were shown to consist only of a ppGpp synthetase domain. Studies suggest that these SAS proteins contribute to ppGpp signalling in response to stressful conditions in a manner distinct from that of RelA-SpoT enzymes. SAS proteins currently appear to always occur in addition to RSH enzymes in various combinations but never alone. RSHs have also been identified in chloroplasts, organelles of photosynthetic eukaryotes that originated from endosymbiotic photosynthetic bacteria. These chloroplast RSHs are exclusively encoded in nuclear DNA and targeted into chloroplasts. The findings suggest that ppGpp may regulate chloroplast functions similar to those regulated in bacteria, including transcription and translation. In addition, a novel ppGpp synthetase that is regulated by Ca²⁺ as a result of the presence of two EF-hand motifs at its COOH terminus was recently identified in chloroplasts of land plants. This finding indicates the existence of a direct connection between eukaryotic Ca²⁺ signalling and prokaryotic ppGpp signalling in chloroplasts. The new observations with regard to ppGpp signalling in land plants suggest that such signalling contributes to the regulation of a wider range of cellular functions than previously anticipated.

Diversity in Guanosine 3′,5′-Bisdiphosphate (ppGpp) Sensitivity among Guanylate Kinases of Bacteria and Plants

“…Targets of ppGpp in bacteria have been found to include RNA polymerase, translation factors, DNA primase, and enzymes of RNA biosynthesis (3,9). Guanylate kinase (GK) was recently identified as a key target of ppGpp in Bacillus subtilis, with the IC 50 for inhibition of GK activity by ppGpp being estimated at ϳ30 M, which is within the concentration range of physiological ppGpp fluctuation in this bacterium (14).…”

Heterologous expression of a plant RelA-SpoT homologue results in increased stress tolerance in Saccharomyces cerevisiae by accumulation of the bacterial alarmone ppGpp

“…Importantly, despite the wide or ubiquitous distribution of relA or RSH in bacteria and plants, neither relA and RSH genes nor ppGpp per se have yet been identified in animals or eukaryotic micro-organisms such as yeasts and fungi. Although the mechanism of ppGpp synthesis is rather poorly understood Tozawa & Nomura, 2011;Tozawa et al, 2007), there is accumulating evidence regarding the function of ppGpp in reorienting cellular metabolism and its physiological consequences, including research into sporulation (Balzer & McLean, 2002;Lemos et al, 2004;Ochi et al, 1981), competence (Inaoka & Ochi, 2002), fruiting body formation (Harris et al, 1998), antibiotic production (Bibb, 2005;Hesketh et al, 2001;Hoyt & Jones, 1999;Inaoka et al, 2003;Ochi, 1987aOchi, , 2007Sun et al, 2001), development of persistence (Dahl et al, 2003;Korch et al, 2003), quorum sensing (Baysse et al, 2005;Harris et al, 1998;van Delden et al, 2001), biofilm formation (Balzer & McLean, 2002), pathogenesis (Erickson et al, 2004;Gaynor et al, 2005;Godfrey et al, 2002;Haralalka et al, 2003;Pizarro-Cerdá & Tedin, 2004;Song et al, 2004) and symbiosis (Moris et al, 2005;Wells & Long, 2002;Zhang et al, 2004). The RNA polymerase is the primary target for ppGpp, as confirmed recently by X-ray crystallographic analysis of an RNA polymerase-ppGpp complex (Artsimovitch et al, 2004;Chatterji et al, 1998;Sato et al, 2009).…”

Cytosolic ppGpp accumulation induces retarded plant growth and development

“…1 Recent progress in genome sequencing has revealed that RelA/SpoT homologs (RSHs) are conserved in plants and green algae. [2][3][4] Moreover, metazoan SpoT homolog-1 (Mesh1) was identified in human and Drosophila melanogaster. 5 Plant RSHs and Drosophila Mesh1 show ppGpp synthase and hydrolase activities, respectively, [5][6][7][8] suggesting that ppGpp functions in eukaryotic cells.…”